Abstract. In order to simulate the light-current (LI) characteristics of the Vertical Cavity Surface Emitting Laser (VCSEL), we adopted the simplex method to determine the optimal solution of the model parameters on the basis of experimental data. Besides, improvement was made to fit the current-voltage (IV) data using the relationship which accounted for a resistance in series with a diode, and it was more appropriate to fit LI curves according to the model evaluation index. Furthermore, the improved model can be applied to fit the LI curves at different ambient temperatures, and it illustrates that maximum value of the output power decreases as the ambient temperature increases.